MSCE Chemistry key terms

The rule that electrons fill atomic orbitals in order of increasing energy, starting with the lowest available orbital, so that a ground-state electron configuration is built up predictably.

The net positive charge an outer electron experiences after inner electrons shield part of the nucleus. It increases across a period and largely explains trends in atomic radius, ionization energy, and electronegativity.

The minimum energy required to remove one electron from a gaseous atom or ion. It generally rises across a period and falls down a group as the outermost electron sits farther from the nucleus.

The energy change when a gaseous atom gains an electron. A large negative value means the atom strongly attracts electrons, which helps predict oxidizing strength and periodic trends.

A relative measure of how strongly an atom pulls shared electrons toward itself in a bond. Differences in this property between bonded atoms predict bond polarity and ionic character.

A bookkeeping value for an atom in a Lewis structure, calculated as valence electrons minus nonbonding electrons minus half the bonding electrons. Structures with charges closest to zero are usually preferred.

The description of a molecule as a weighted average of several valid Lewis structures that differ only in electron placement. The real structure is a hybrid, not a rapidly interconverting mixture.

A model predicting molecular shape by arranging electron domains around a central atom so they repel as far apart as possible. Lone pairs occupy more space than bonding pairs, compressing bond angles.

The mixing of atomic orbitals on a central atom to form equivalent hybrid orbitals that match the observed geometry, such as sp3 for tetrahedral and sp2 for trigonal planar arrangements.

A vector quantity equal to charge separation multiplied by distance, indicating the direction and size of bond polarity. Molecular dipoles are the vector sum of individual bond dipoles.

The reactant that is completely consumed first and therefore determines the maximum amount of product formed. Identifying it requires comparing the mole ratio supplied with the stoichiometric ratio.

The maximum mass or amount of product calculated from stoichiometry when the limiting reagent is fully converted. Percent yield compares the actual isolated amount with this value.

The SI amount of substance containing Avogadro's number of elementary entities, approximately 6.022 times ten to the twenty-third. It links mass, particle count, and gas volume calculations.

Concentration expressed as moles of solute per liter of solution. Because solution volume changes with temperature, molality is preferred when temperature varies.

Concentration expressed as moles of solute per kilogram of solvent. It is temperature-independent and is used in boiling-point elevation, freezing-point depression, and some equilibrium calculations.

A solution property depending on the number of dissolved particles rather than their identity, including vapor-pressure lowering, boiling-point elevation, freezing-point depression, and osmotic pressure.

A thermodynamic quantity representing heat exchanged at constant pressure. Its change, delta H, is negative for exothermic reactions and positive for endothermic ones.

The principle that the enthalpy change of a reaction is independent of the pathway, so it equals the sum of enthalpy changes for any sequence of steps that connects reactants to products.

A combined thermodynamic function, G equals H minus T times S, whose change determines spontaneity. A negative delta G indicates a spontaneous process at constant temperature and pressure.

A measure of energy dispersal or disorder in a system. The second law states that the total entropy of the universe increases for any spontaneous process.

When a system at equilibrium is disturbed by a change in concentration, pressure, or temperature, it shifts in the direction that partially offsets the disturbance.

The ratio of product activities to reactant activities at equilibrium, each raised to its stoichiometric coefficient. Its value depends on temperature and indicates the reaction's extent.

The same ratio as the equilibrium constant but calculated with current, nonequilibrium concentrations. Comparing it with the equilibrium constant predicts the direction of net change.

The amount of strong acid or base a buffer can absorb before its pH changes appreciably. It is greatest when the buffer ratio is near one and when total concentrations are high.

The point in a titration where the amount of titrant added exactly matches the stoichiometric amount needed to react with the analyte. It is detected by an indicator or instrumental signal.

The observed point at which a titration is stopped, usually signaled by an indicator color change or a sudden sensor response. It should be as close as possible to the equivalence point.

The slowest elementary step in a reaction mechanism, which controls the overall reaction rate. Its stoichiometry and energetics appear in the experimental rate law.

The minimum energy colliding molecules need for a reaction to occur. It equals the barrier between reactants and the transition state and appears in the Arrhenius equation.

A substance that speeds a reaction by providing an alternative pathway with lower activation energy while remaining chemically unchanged overall. It does not alter the equilibrium constant.

The voltage of a half-cell measured against the standard hydrogen electrode under standard conditions. More positive values indicate a greater tendency to be reduced.

A relation giving cell potential as the standard potential minus a term depending on the reaction quotient. It lets you calculate voltages at nonstandard concentrations and link potential to equilibrium.

An electron-rich species that donates a pair of electrons to an electron-poor center, forming a new bond. Common examples include hydroxide, alkoxides, amines, and carbanions.

An electron-poor species that accepts an electron pair from a nucleophile. Carbocations, carbonyl carbons, and halogens acting as Lewis acids are typical examples.

A one-step nucleophilic substitution in which the nucleophile attacks as the leaving group departs, giving inversion of configuration at the carbon center and second-order kinetics.

The substituent displaced from a substrate during substitution or elimination. Good leaving groups are weak bases, such as iodide or tosylate, that stabilize the negative charge well.

A separation technique in which components partition differently between a mobile phase and a stationary phase, so they migrate at different rates and emerge separated.

The time a component takes to travel from injection to detector in a chromatographic run. It is used for qualitative identification when compared with a known standard under identical conditions.

A known amount of a compound added to every sample and calibration standard to correct for variations in injection volume, extraction recovery, and instrument response during quantitative analysis.

A plot of instrument response against known analyte concentrations, used to determine unknown amounts. Its slope gives sensitivity and its linear range defines valid quantitation.

The lowest analyte amount that can be reliably distinguished from background noise, typically three times the standard deviation of blank measurements divided by the calibration slope.

An ion or molecule that donates one or more electron pairs to a central metal ion, forming coordinate bonds. Ligands are classified by denticity, such as monodentate or bidentate.

The enhanced stability of complexes formed by multidentate ligands compared with similar monodentate ligands, largely because ring formation reduces the entropy penalty of binding.

A model treating metal-ligand interactions as electrostatic, splitting d-orbital energies by ligand geometry. The size of the splitting explains color and magnetic behavior of complexes.

A standardized document supplied with a hazardous chemical that lists its hazards, first-aid measures, handling and storage precautions, exposure limits, and disposal requirements.

A written program required in laboratories that sets out work practices, engineering controls, personal protective equipment, and procedures to protect workers from hazardous chemicals.

A ventilated enclosure that draws air away from the worker to control exposure to vapors, dusts, and fumes. Its sash position and face velocity must be checked before use.

The selection of protective gloves whose material resists permeation and degradation by the specific chemicals handled. No single glove material protects against all substances.